The Dig Allen books were, as far as I know, printed in three distinct formats:

The first Dig Allen book (The Forgotten Star) was printed as a hardbound book with a dustjacket. (Click here to see a picture of it.) This was the only Dig Allen book to have a dustjacket.

All six Dig Allen books were printed as hardback books with pictures on the front cover of the book.

A few Dig Allen books were printed as paperback books with highly unusual cover art. So far I have located paperback copies of The Forgotten Star (click here for a picture of one) and Captives in Space (click here for a picture of one). I think that the other four books in the series were also issued in paperback format, but I'm not completely sure. (If you know of a paperback copy of a Dig Allen book other than the first two, please let me know! I can be reached at ednacooper@compuserve.com.)

Which formats are the most common? Well, for The Forgotten Star, I have found that the dustjacket format is far more common than the pictorial cover format. For the other five volumes, the pictorial cover format is the most common.

As for the paperback copies, those are quite difficult to find! Only rarely does a paperback Dig Allen book surface; however, when one does surface it usually only sells for a couple dollars. I would say, then, that the paperback Dig Allen books are by far the most difficult format to find.